The present invention relates to a fuel system which serves to supply fuel stored in a fuel tank to a fuel injection valve through a fuel supply pipeline by means of a fuel pump and to return surplus fuel from a fuel injection pressure regulator serving to regulate the fuel injection pressure of the fuel injection valve to a specified pressure to the fuel tank through a fuel return pipeline.
In general, the fuel system of an internal combustion engine comprises a fuel tank, a fuel pump, a fuel injection valve and a fuel injection pressure regulator, so as to serve to supply fuel to the fuel injection valve through a fuel supply pipeline and to return surplus fuel from the fuel injection pressure regulator to the fuel tank through a fuel return pipeline.
In this kind of fuel system, owing to evaporation from the surface of fuel stored in the fuel tank or owing to abrupt change of fuel pressure caused by the fuel injection pressure regulator, fuel vapor is generated. For this reason, there is provided in an automobile such an apparatus or the like that controls exhaust of the fuel vapor by adsorbing the generated fuel vapor on activated carbon.
However, the fuel vapor leaks out in some cases to cause air pollution. To cope with this, it has been considered to restrain generation of fuel vapor itself, but there are the following problems.
In order to reduce the fuel vapor generated by evaporation, a fuel cooling system disclosed in for example Japanese Patent Unexamined Publication No. 61-101660 can be used. However, the fuel vapor resulting from abrupt reduction of pressure (overshoot) is generated with deaeration phenomenon, so that it is not so influenced by the temperature and, hence, the fuel cooling system described above has little reducing effect. Further, there has not been proposed means for reducing fuel vapor by preventing such deaeration phenomenon.
Japanese Utility Model Unexamined Publication No. 61-36139 discloses a technique for controlling the pressure of fuel which is returned from a fuel injection valve to a fuel tank. However, in a fuel pressure regulating device disclosed in this publication, the set pressure of a fuel injection pressure regulator is changed to improve the restarting ability of an engine. Therefore, even by making use of such device, the same overshoot as described above is caused to fail to reduce the fuel vapor effectively.
An object of the present invention is to provide a fuel system which is capable of reducing fuel vapor effectively.